CONFMAN.edit_payment = function() {

  var discount = $('discount');
  var amount_paid = $('amount_paid');

  var change_total = function(box) {
    var amount = trim(box.getAttribute('data-amount'));
    switch(trim(box.getAttribute('data-type'))) {
      case "Discount": {
        if (box.checked) {
          discount.value = (parseFloat(discount.value) - parseFloat(amount)).toFixed(2);
        } else {
          discount.value = (parseFloat(discount.value) + parseFloat(amount)).toFixed(2);
        }
        break;
      }
      default: {
        if (box.checked) {
          amount_paid.value = (parseFloat(amount_paid.value) - parseFloat(amount)).toFixed(2);
        } else {
          amount_paid.value = (parseFloat(amount_paid.value) + parseFloat(amount)).toFixed(2);
        }
        break;
      }
    }
  };

  var trim = function(str) {
    return str.replace(/^\s+|\s+$/g, "");
  };

  $$('input[type=checkbox]').invoke('observe', 'click', function(e) {
    var elem = e.findElement('input[type=checkbox]');
    if (elem !== null) {
      change_total(elem);
    }
  });

};